Technical Abstract:
Rainfall simulations are widely used to evaluate pesticide runoff potential. Most are conducted at constant intensity, i.e. at a fixed rainfall rate. In contrast large within storm intensity variation in natural rainfall is common. To assess implications, field-based simulations were conducted. Runoff expressed as percent of active ingredient applied was evaluated stepwise by simulation pattern: constant versus variable; tillage: strip- (ST) versus conventional (CT); and pesticide transport characteristics: aqueous phase dominated (fluometuron) versus sediment associated (pendimethalin). The variable pattern was representative of spring storms in the southern Atlantic Coastal Plain of Georgia (USA) where the work was conducted. The herbicides are commonly used by cotton growers. For both compounds under both tillages, simulation intensity pattern strongly impacted runoff kinetics. However, event-based runoff totals were not significantly different with one exception, fluometuron runoff from CT plots. The variable yielded about 25% more than the constant intensity pattern (P=0.10). This was linked to fluometuron’s tendency to leach and because soil was freshly tilled and relatively dry when simulations were conducted. Tillage and herbicide transport characteristics had a greater impact on total runoff. CT plots delivered 8 to 10 fold more pendimethalin in runoff than ST plots. Mean differences were significant (P=0.05). This was also case when pendimethalin and fluometuron runoff from CT plots was compared. An opposite trend was observed on ST plots. Fluometuron was greater than pendimethalin runoff. Fluometuron runoff was also greater from ST than CT plots; but differences in response were not significant due to high variability in fluometuron ST plot runoff. The study indicates that use of constant versus variable intensity rainfall in simulations will not substantially impact runoff trends with the possible exception of relatively water soluble compounds like fluometuron under CT conditions.